One of the main differences between the 4x and PRO is that the PRO can register up to 8 bases as one big group while the 4x base will always remain a stand alone system. Meaning, if you have two 4x base stations, they cannot work together with each other when the PRO can.
Because of this the air protocols are different and being that they are different, they won't work with each other. So even though they use the same form factor and even the same chip set inside, because of the air protocol differences, they cannot work with each other. An analogy would be bluetooth and WiFi. They both use 2.4GHz but how and what protocols they are tranmitting are different. The analogy isn't exactly fair because the 4x and PRO are actually much similar than that the example, but its different enough to not be compatible.